For the past two weeks, Europe has been grappling with a blistering heatwave that scientists have called the worst extreme heat event on record for the continent, made possible only by climate change. Now, a jaw-dropping new assessment of its death toll underscores the dangers of our rapidly warming planet. In a study that has yet to be peer-reviewed, Christopher Callahan, a climate scientist and assistant professor at Indiana University Bloomington, estimates 20,390 heat-related deaths across Europe between June 22 and 28. Callahan’s analysis, currently available on the preprint server Zenodo, is based on a previously developed statistical model that assesses how mortality rate changes as temperatures rise across Europe. He believes his results are qualitatively consistent with on-the-ground reports of excess mortality since the heat wave began.
In France—which recorded its hottest day on record on June 23—health authorities reported more than 1,000 excess deaths between June 24 and 27, and funeral homes in Paris have become overwhelmed, according to the Agence France-Presse. “Europe is the fastest-warming continent on Earth, heating at twice the global average,” World Health Organization Director-General Tedros Adhanom Ghebreyesus posted on X Sunday. “Right now 150 million people are living under extreme heat, hundreds have died, schools are shut, grids are buckling.”
Heat is the deadliest natural hazard in Europe New research by World Weather Attribution, an international scientific organization that studies how climate change influences the likelihood and intensity of extreme weather events, shows that heatwaves cause more deaths in Europe than all other natural hazards combined. In 2003, the first major heatwave of this century is estimated to have killed more than 70,000 people across the continent. Since then, adaptation measures have led to mortality reductions in later summers such as 2006, but the potential for mass heat mortality remains substantial in an ever-warming climate, according to Callahan.
To estimate the continent-wide death toll of the June heatwave, he used an established statistical model that assesses heat-related mortality across more than 900 subnational regions of Europe between 2015 and 2019. The model estimates how weekly death rates change as temperatures rise while accounting for normal seasonal patterns and regional differences. The model showed that mortality risk rises rapidly as daily maximum temperatures increase. Days above 104 degrees Fahrenheit (40 degrees Celsius) cause weekly mortality rates to rise more than 6% compared to days around 77 degrees F (25 degrees C). The findings suggest that warmer regions are less vulnerable to heat, consistent with evidence of adaptation. But when temperatures rise above 104 degrees F (40 degrees C), warmer regions experienced greater mortality risk, suggesting the effectiveness of adaptation efforts may be limited by extreme temperature rise.
Callahan then applied this fitted model to the June 2026 heatwave, comparing weekly temperatures with their corresponding averages based on climate data from 1991 to 2020. This allowed him to estimate the excess mortality linked to this event, finding that the death toll exceeds 20,000. “The vast majority of the deaths likely occurred during the week of June 22-28, when temperatures peaked at above (104 degrees F) 40 degrees C across many countries,” the study states. “Over space, the greatest mortality was estimated in France (5,210), Germany (4,543), Spain (3,163), and Italy (2,709).”
Approaching the limits of climate adaptation It bears repeating that Callahan’s finding is a statistical estimate based on preliminary data, not an official count of excess deaths across Europe. What’s more, his analysis not yet been evaluated by other scientists. Still, he believes his study “highlights the accelerating threat of mass heat mortality events in Europe despite previous adaptations to heat.” As Earth’s climate continues to warm, events like this will only become more frequent and intense. The study by World Weather Attribution showed that extreme heatwaves are increasing rapidly, with such events tens to hundreds of times more likely since 2003 and virtually impossible just 50 years ago. “Every ton of CO2 emitted appreciably increases global average temperature and thus local heat extremes,” Callahan states in his report. “Given the continued increase in fossil fuel emissions and the potential for extreme events even if humanity undertakes rapid decarbonization, significant increases in adaptation investments will be necessary to reduce future heat mortality.”
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